1. Field of the Invention
This invention relates to the press fabrics used in the press section of papermaking machines to support, carry, and dewater the wet fibrous sheet as it is being processed into paper. It more specifically relates to open-ended fabrics whose ends are joined by means of a pin seam when being installed on the machine. The invention further relates to the use of a braided yarn for the machine direction (MD) strands of the press fabric.
2. Background Information
Endless fabrics are key components of the machines used to manufacture paper products. Of immediate concern here are the fabrics used in the press section. Not only does the press fabric act as a type of conveyor belt carrying the wet fibrous sheet being processed into paper through the section, but, more importantly, it also accepts water that is mechanically pressed from the sheet as it passes through the press. More specifically, the press squeezes water from the sheet into the fabric.
Until fairly recently, the press fabrics used in the press section were supplied in endless form; that is, they were woven in the form of an endless loop without a seam. This was partly because of the limitations of seam and weaving technology. In addition, however, the press section poses additional special considerations not present in the other sections of the papermaking machine.
Historically, most of the methods of joining the ends of an open fabric involve a seam which is much thicker than the rest of the body of the fabric. This can cause major problems on a fabric used in the press section. The thicker seam will be subjected to higher compressive forces on each passage through the press nip weakening the seam and thus shortening fabric life. In addition, potentially damaging vibrations can be set up in the press machinery by the repetitive passages of the thicker seam region. Finally, the wet fibrous sheet, still quite fragile in the press section because of its high water content, can be marked, if not broken, by extra compression at the seam location.
Despite these considerable obstacles, it remained highly desirable to develop an on-machine-seamed (OMS) press fabric, because of the comparative ease and safety with which it can be installed on the machine. This simply involves pulling one end of the open-ended press fabric through the machine, around the various guide and tension rolls and other components. Then, the two ends can be joined at a convenient location on the machine and the tension adjusted to make the fabric taut. In fact, a new fabric is usually installed at the same time as an old one is removed. In such a case, one end of the new fabric is connected to an end of the old fabric, which is used to pull the new fabric into its proper position on the machine.
By way of contrast, the installation of an endless fabric on a press section is a difficult and time-consuming undertaking. The machine must be shut down for a comparatively longer period while the old fabric is cut out or otherwise removed. The new fabric then must be slipped into proper position from the side into the gaps between the presses through the frame and around other machine components. The difficulty of this procedure is further compounded by the fact that the newer - generation press fabrics are gradually becoming thicker and stiffer. These characteristics add to the time and effort required on the part of plant personnel to install a new one. In this connection, a workable OMS press fabric was an advance long sought by the industry.
One method of joining the ends of an open-ended fabric together is by using a pin seam, so called because its integral element is a pin, or pintle, which joins together the loops at the ends of the press fabric.
One method to produce an open-ended fabric, that can be joined on the paper machine with a pin seam, is to weave the fabric in such a way that the ends of the machine direction (MD) strands can be turned back and woven into the body of the fabric and parallel to the machine direction. The second technique employs the art of weaving "endless", which normally results in a continuous loop of fabric. However, when making a pin-seamable press fabric, one edge of the fabric is woven in such a way that the body yarns form loops, one set of alternating loops for each end of the woven cloth.
The ends of the fabric are joined by bringing them into close proximity with each other, intermeshing and alternating the loops on each end of the fabric. The pintle is then passed through the voided space running down the centers of the intersecting loops to complete the seam. The region of the seam is only slightly thicker than the main body of the fabric belt, because the loops formed use the MD body yarn strands.
The present invention concerns the problems with the loops themselves. The MD yarn in a conventionally woven press fabric structure, flat or endless, has not previously had the added requirements of loop formation and integrity.
Single monofilament was originally used in the machine direction for OMS press fabrics since it was stiff and has good loop formation properties. But experience showed it not only to be difficult to weave but also to have insufficient MD elasticity for many kinds of contemporary presses. As a consequence, tensile failure and seam breakage have been problems.
Standard textile ply/twisted monofilament has been used in the machine direction in an attempt to solve these problems. In the weaving process, it has proved to be much easier to use than single monofilament. Its improved elasticity and strength answer the tensile and fatigue problems of single monofilament. However, when one attempts to form the loops for a pin seam from these MD yarns, serious problems are encountered. The loops so formed have the tendency to deform at the apex. In addition, the entire loop will rather easily deform or bend as one attempts to force a pintle through the loop opening.
Another problem arises as a result of a phenomenon called the secondary helix effect. It will be recalled that ideally the pin seam loops will be properly oriented when their planes are perpendicular to the plane of the fabric and parallel to the machine direction. Such an orientation makes it possible for the loops at each end of the fabric to be intermeshed and alternated easily during the joining of the ends to form a pin seam. The secondary helix effect is observed in the tendency of a loop formed from a twisted yarn to turn about an axis lying in the plane of the loop. When this occurs, it represents a departure of the loop from the ideal orientation needed to form the pin seam. Such departure makes it difficult, if not impossible, to properly intermesh and alternate the loops on each end of the press fabric during closure, as well as to force the pintle through the void created by the intermeshed loops.
This invention represents a means to overcome this difficulty. The OMS loops formed as instructed here will have the tensile strength and fatigue resistance of twisted monofilament, yet will not exhibit the twisting behavior illustrative of the secondary helix effect.